The present invention relates to Film Bulk Acoustic Resonator (FBAR) for thin film filter, and more particular to a FBAR device and a method of fabricating thereof, having robust structure and simplified process.
Recently, the development of an electrical communication technology has been accompanied by the development of signal processing technology and radio frequency (RF) device fabrication.
The need to reduce the cost and size of electronic equipment has led to a continuing need for smaller RF device. The miniaturization of RF device, such as FBAR device, has been accomplished through semiconductor process technology of fabricating a thin film.
The film bulk acoustic resonator (FBAR) device of various RF devices is a filter embodied by thin film type device, which has the piezoelectric material layer on wafer and can cause a resonance effect due to the piezoelectricity thereof.
Typically, the FBAR device comprises a first electrode, a piezoelectric layer and a second electrode, which are formed sequentially on a substrate, such as Si substrate. On applying electric field between the first electrode and the second electrode, the piezoelectric layer generates an acoustic wave. However, the acoustic wave must be separated from the substrate effect to maintain high quality factor (Q). Therefore, the FBAR device requires an isolation structure that separates the resonance region, including the first electrode, the piezoelectric layer and the second electrode, from the substrate, for preventing the acoustic wave from being irrupted by the substrate.
Thus, the performance of the FBAR device and the practicality of the fabrication method depend on how to fabricate such isolation structure.
In the recent FBAR devices, the isolating methods of separating the resonance region from the substrate include an air gap forming method using an etching cavity and a reflecting method using a Bragg reflecting film.
FIGS. 1A-1C illustrate cross-sectional structural views exemplary of a FBAR device structure in accordance with the prior method.
Referring to FIG. 1A, FBAR device according to the prior art comprises a Si substrate 10, a membrane layer 14, a first electrode 16, a piezoelectric layer 18 and a second electrode 20. Typically, the membrane layer includes SiO2. In particular, the substrate has an etching cavity, which is formed by performing anisotropic etch to an etch stop. However, this FBAR structure is very weak and this method has low yield.
Referring to FIG. 1B, a FBAR device according to another air gap method is illustrated in FIG. 1B, which is also called as Air-Bridge method. This method comprises forming a sacrificial layer, which will be formed into air gap 12 in FIG. 1B, on a portion of the substrate surface 10, forming an insulator layer 14 on the substrate 10 and the sacrificial layer, forming a first electrode 16, a piezoelectric layer 18 and a second electrode 20 sequentially on the insulator layer 14 and then removing the sacrificial layer by performing wet-etch process through a via hole (not shown). The resulting structure may cause collapse or unwanted delamination during the subsequent process, such as removing the photoresist and slicing the resultant wafer.
Referring to FIG. 1C, in contrast with FIGS. 1A and 1B, it illustrates a FBAR device fabricated according to a reflecting film method, which is also called as Solidly Mounted Resonator (SMR) method. This method comprises depositing reflecting layers 22 and 23 alternatively, of which the acoustic impedance difference is large, and forming a first electrode 16, a piezoelectric layer 18 and a second electrode 20 sequentially thereon. In this method, the difference of acoustic impedances is used to generate large acoustic impedance from the lower part of the device, resulting in separating the resonance region from the substrate. However, in step of depositing reflecting layers 22 and 23 alternatively, each of layer thickness is required to be controlled accurately and have xcex/4 of resonance frequency. Thus, such controlling in SMR is very difficult and time-consuming. Further, the FBAR device according to SMR has lower reflectivity than one according to the air gap method, and has also the problem of reducing effective bandwidth.
Therefore, an improved FBAR device and method have been required to overcome the limitation of the prior methods. The present invention provides a FBAR device and a method of fabricating thereof, having robust structure, good reflectivity and stable effective bandwidth by simplified process.
It is an object of the present invention to provide a robust FBAR device and a simplified method of fabricating a FBAR device. FBAR device according to the present invention includes a membrane supporting layer between a substrate and a membrane layer surrounding an air gap region. The membrane supporting layer supports the membrane layer to obtain a strong structure.
The method according to the present invention first forms a sacrificial layer on the substrate, then a photoresist pattern is formed on air gap forming region at a top surface of the sacrificial layer, the method removes the sacrificial layer to form a sacrificial pattern by using the photoresist pattern as an etching mask. An insulating material then deposits on the substrate, the photoresist pattern is remove, and a membrane layer is formed on a top surface of the sacrificial layer and the insulating material layer. Finally, the method removes the sacrificial pattern to form air gap.
In an preferred embodiment of the present invention, said step of etching the sacrificial layer may include over-etching the sacrificial layer to form the sacrificial pattern having an undercut and also include controlling the width of the undercut by using dry etch after forming the sacrificial pattern.
In another preferred embodiment of the present invention, the method may further comprise the step of controlling edge profile of the sacrificial pattern by hard baking the photoresist pattern with a hot plate, after forming the photoresist pattern but before forming the sacrificial pattern.